Recently, so-called PAN-based carbon fibers using polyacrylonitrile (PAN) as a starting material, so-called rayon-based carbon fibers using rayon as a starting material, and so-called pitch-based carbon fibers using pitches as a starting material are manufactured as carbon fibers. Moreover, as pitch-based carbon fibers, anisotropic carbon fibers and isotropic carbon fibers are manufactured. Among them, pitch-based isotropic carbon fibers are widely used for usages in which the characteristics such as lightness, chemical resistance, heating resistance, sliding ability and electrical conductivity are demanded, because pitch-based isotropic carbon fibers are comparatively economic due to starting material of low price, and can be manufactured by a method suitable for mass production.
Moreover, carbon fibers are used in various forms such as a filament, a sliver (a fiber bundle), a spun yarn, a fabric, a chopped fiber, a milled fiber, a mat, and a prepreg. Further, a heat treatment temperature and a degree of carbonization for manufacturing carbon fibers can be adjusted depending on the usage. Among them, carbon fiber spun yarns and carbon fiber fabrics are used as composition materials such as a heat insulation, a sliding member, a conductive material, or the like, especially, used as the electronic industry materials such as a gas diffuser for a solid polymer electrolyte fuel cell.
Therefore, such carbon fiber spun yarns and carbon fiber fabrics are demanded that they have not only flexibility and electrical conductivity but also gas permeability, adhesion to a matrix material such as polymer materials, uniformity of fineness and thickness, high tensile strength, and the like.
The carbon fiber spun yarns and carbon fiber fabrics can have a high electrical conductivity by heat-treating them at a high temperature of 900° C. or higher, to improve their degree of carbonization.
Further, when the carbon fiber fabrics are used as a gas diffuser for a solid polymer electrolyte fuel cell, the gas permeability depends on the aperture ratio (porosity). However, when a too-rough porous body is used as the gas diffuser, contact with the catalyst layer becomes poor, so that a problem with electric power collection is occurred. Moreover, for the filament fabric in which single yarns are aligned, there is a problem that the aperture ratio (porosity) is small and the gas permeability is low. Therefore, as such carbon fiber fabrics, spun yarn fabrics are still preferable than filament fabrics in which the single yarns may be aligned and of which the density thereof becomes high easily. Moreover, it is necessary to appropriately control the thickness of the carbon fiber fabrics used as a gas diffuser in consideration of the diffusion of a reactive gas to the catalyst layer.
Therefore, carbon fiber fabrics used as a gas diffuser are preferably spun yarn fabrics having an appropriate thickness and having a heat history of 900° C. or higher. As a method for obtaining such spun yarn fabrics, a method of weaving a fabric using spun yarns of flame-resistant fibers or carbonaceous fibers, and then heat-treating the resultant fabric at a temperature of 900° C. or higher, or a method of heat-treating spun yarns at a temperature of 900° C. or higher, and then weaving a fabric using the heat-treated spun yarns, is adopted.
As such carbon fiber spun yarns, PAN-based spun yarns having flame-resistant and pitch-based spun yarns are known. However, it is difficult to weave a fabric using the PAN-based spun yarns of flame-resistant because the strength thereof decreases extremely when they are heat-treated at 900° C. Therefore, if the method of weaving a fabric using the flame-resistant fibers, and then heat-treating the resultant fabric at a temperature of 900° C. is not adopted, the objective fabric cannot be obtained. However, in that case, there has been a fault that strength of the resultant fabric lowered due to the fiber distortion by heat-treatment and the strength decrease of the spun yarn by heat-treatment. Moreover, if a carbon fiber having a long fiber length such as a PAN-based continuousness carbon fiber is used, there is a problem that the adhesion of the resultant spun yarn to a matrix material is inferior.
Therefore, Japanese Unexamined Patent Application Publication No. 2002-352807 (document 1), for instance, discloses a gas diffuser comprising a porous carbon substrate that is formed by uniting carbon fibers having a fiber length of 25 to 80 mm with air-gaps, and that contains a particulate fluororesin in an amount of 1 to 40 wt %, wherein the particulate fluororesin binds the carbon fibers together with each other as a binder. Moreover, Japanese Unexamined Patent Application Publication No. 2003-288906 (document 2) discloses a gas diffuser having a carbon layer on at least one side of a carbon fiber fabric for an electrode, wherein the carbon layer includes carbon black and a fluororesin. However, in the carbon fiber spun yarns described in document 1 and document 2, tensile strength thereof is low. For this reason, the gas diffusers fabric comprising them have a problem of an inevitable decrease of electric power collection ability due to a reason that the particulate fluororesin which is a binder or the carbon layer is used to improve tensile strength of the resultant carbon fiber fabric.
Further, in Japanese Unexamined Patent Application Publication No. Sho 53-81735, there is proposed to obtain carbon fiber spun yarns having an improved strength by spinning sliver-shaped carbon fibers having a fiber length of 25 mm or longer, more preferably in a range of 50 to 75 mm. However, in the carbon fiber spun yarns thus obtained, the tensile strength thereof is in a range of around 0.08 to 0.09 N/tex, and it is not enough yet.
On the other hand, the majority of pitch-based isotropic carbon fibers are those having a short fiber length. Moreover, spun yarns which are produced by using pitch-based isotropic carbon fibers and improving carbonization thereof are marketed. However, carbon fiber spun yarns having a sufficient tensile strength as well as a low fineness fluctuation cannot be commercially available. Therefore, fabrics woven using commercially available carbon fiber spun yarns are not enough in the point of strength and the thickness fluctuation.